1. Field of the Invention
The present invention relates to an optical element using an optical anisotropic medium, and an optical apparatus such as a projector which uses the optical element.
2. Description of the Related Art
The optical element that uses the optical anisotropy has been in wide use for polarization control, beam separation, or the like. For example, the optical element is used as a phase plate represented by a λ/4 plate or a λ/2 plate, a polarization plate represented by a polarization separation element, or an optical low-pass filter.
The optical anisotropy means a nature of variance of refractive indexes depending on a vibration direction of an incident polarized light. The use of this nature enables variance of behaviors depending on polarization directions even in the case of lights entering from the same direction.
Materials having such optical anisotropy include a crystal material such as a crystal or a limestone, a liquid crystal material, and an organic material such as plastic or a high molecule. A degree of optical anisotropy of such a material is represented by a refractive index with respect to a polarization direction.
Japanese Patent Laid-Open Nos. 2004-139001 and 2007-156441 disclose, as a method for obtaining optical anisotropy, methods which use structural anisotropy based on structures smaller than a wavelength of a used light (hereinafter referred to as a used wavelength).
In the structure smaller than the used wavelength, a light is known to behave like a homogeneous medium without being able to directly recognize the structure. In this case, the light exhibits a refractive index compliant with a filling rate. The refractive index can be obtained by a method called an effective refractive index method. A nature of variance of refractive indexes depending on polarization directions according to the filling rate of the structure is called structural anisotropy. Optionally setting a filling rate of the structure enables adjustment of a refractive index. The use of structural anisotropy enables an increase of a difference of refractive indexes depending on polarization directions as compared with a normal optical anisotropic material. Thus, a thickness for obtaining desired birefringence characteristics can be reduced.
Japanese Patent Laid-Open No. 2004-139001 discloses a phase plate which uses structural anisotropy. In the phase plate, by using the capability of the structural anisotropy to adjust the refractive index, a plurality of structural anisotropic layers (periodic structures) are combined to suppress changes in phase difference characteristics caused by wavelengths.
Japanese Patent Laid-Open No. 2007-156441 discloses an optical element which includes a structural anisotropic layer of a plane normal direction formed in one surface of a substrate, and a structural anisotropic layer of an in-plane direction in the other surface. A phase compensation plate is obtained by adjusting a refractive index based on the structural anisotropy of each surface and combining the refractive indexes. An antireflection coating is inserted to provide an antireflective function.
Japanese Patent Laid-Open No. 2004-139001 discloses an example where a material of a low refractive index having similar periodicity is disposed on a structural anisotropic layer using a medium of a high refractive index. Thus, since the material of a refractive index lower than that of each structural anisotropic layer is stacked thereon, reflection on the surface is suppressed to a certain extent. With this configuration, however, antireflective performance is insufficient.
In Japanese Patent Laid-Open No. 2007-156441, an antireflection coating is provided to a structural anisotropic layer. However, no configuration necessary for exhibiting an antireflective function is disclosed.
When the material of large refractive index variance depending on polarization directions is used, reflection-transmission characteristics greatly vary with respect to the polarization directions. Even when antireflective coating is provided, because of the large variance of refractive indexes, optimization of characteristics for each polarization direction is difficult.